Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless devices, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a wireless communications network and/or cellular communication system, sometimes also referred to as a cellular radio system or cellular network. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or tablets with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a radio network node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated at the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations or radio nodes communicate over the air interface operating on radio frequencies with the communication devices, also denoted wireless devices, within range of the base stations or radio nodes. In the context of this disclosure, the expression Downlink (DL) is sometimes herein used for the transmission path from the radio node, e.g. a base station, to the wireless device. However, it should be understood that DL may sometimes herein be used for the transmission path from a node controlling the radio interface to the wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the radio node. Further, UL may sometimes be used for the transmission path from the wireless device to the node controlling the radio interface. The radio network node may in some circumstances, e.g. in systems enabling device-to-device (D2D) communications, also be another wireless device. A transmission path in a D2D communication is a transmission path between two nodes, which nodes are not in control of the radio interface.
Mobile data traffic is growing exponentially due to the enormous success of smart phones, tablets and other data traffic appliances. The traditional way for increasing the data rate have been to increase the transmission bandwidth. However, the spectrum has become scarce due to the increase in wireless communications systems and hence the main challenge for the future wireless communications systems is to find alternative solutions to meet high demands on the data rate. One way of handling the increased wireless data traffic is to deploy more base stations and densify the wireless communications systems. This would however increase interference and deployment cost. Another option for increasing the data rate is to introduce large antenna arrays at the base station. Such an option seems to be simpler in terms of deployment cost. The base station, having an excessive number of antennas, can simultaneously schedule multiple terminals at the same time-frequency band with simple linear processing such as Maximum-Ratio Transmission (MRT) or Zero-Forcing (ZF) in the downlink and Maximum-Ratio Combining (MRC) or ZF in the uplink. This is often referred to as massive Multi-User (MU) Multiple-Input-Multiple-Output (MIMO), and is abbreviated by massive MIMO hereafter.
The biggest challenge in deploying massive MIMO is how to acquire Channel State Information (CSI) which is very essential to gain the potentials of the excessive amount of transmit antennas at the base station. Traditionally, each terminal, thanks to the pilot symbols transmitted during downlink phase, estimates the channel gain and feeds it back to the base station via a reverse link. Since the number of required pilots in the downlink is proportional to the number of base station antennas, these schemes for obtaining CSI might require a fair amount of signaling overhead. The idea is therefore to operate in the Time-Division Duplex (TDD) mode and rely on the channel reciprocity between the uplink and the downlink. More precisely, each terminal transmits pilot symbols in the uplink phase, which are then used by the base station to estimate the channel in both directions. The amount of required pilots is thus equal to the number of active terminals, which is typically much smaller than the number of base station antennas for simultaneous data transmissions. But for the number of connected terminals to the base station, the number of users can be much larger than the number of antennas. This, on the other hand, introduces a new challenge in assigning a limited amount of UL-pilots to different users. Many of which users will not need to do any data transmissions, but the wireless communications system might still need CSI for these users, for example, to enable fast activation.
One fundamental assumption in MU-MIMO and massive MIMO is that the base station can acquire sufficiently accurate CSI to the terminals. Then it can perform coherent downlink beam-forming based on the acquired CSI. Many different kinds of CSI acquisition and beam-forming techniques can be found in the literature. Consider a base station with M antennas that serves K terminals, where each terminal has a single antenna. Let hk be an M-vector that represents the channel response of terminal k in a particular resource block. Within one resource block, also called a coherence interval in the massive MIMO literature, the channel is roughly constant over the time-frequency space. The length T of the resource block in number of symbols is assumed equal or smaller to the coherence time multiplied with the coherence bandwidth. Then, in the downlink, the base station transmits a linear combination of the beam-formed signal vectors at each sample time t:Σk=1Kak√{square root over (γk)}sk(t)  (1)where {ak} are beam-forming vectors associated with each of the K terminals, {γk} are the corresponding power control parameters, and sk(t) are symbols intended for terminal k. The beam-forming vectors {ak} are chosen as functions of the (estimated) channel responses {hk} to maximize performance. Within a coherence block, up to T downlink data symbols {sk(1), . . . , sk(T)} can be conveyed to each terminal k, but some symbols are typically reserved for other purposes.
For large antenna arrays, coherent beam-forming is used by the base station, e.g. a Radio Network Node (RNN), array to focus the emitted power onto the specific geographical positions of the terminals, e.g. the wireless devices. In practice the beam-forming operation requires that the RNN acquires information of the channel responses to the wireless devices and new estimates are required roughly once in every coherence block due to natural channel variations. In Time-Division Duplex (TDD) systems, this is typically done by sending uplink pilots in each resource block to estimate the current channel responses to the wireless devices. This requires that each wireless device uses a unique pilot. The number of orthogonal pilots is limited by the amount of time-frequency resources spent on the pilot transmission, which is fundamentally limited by the number of symbols, T, per resource block. In practice this channel knowledge gives the antenna beam-forming gain for the wireless device.
The pilot transmissions also add a pre-log penalty to the rate performance, in the sense that not all symbols in a coherence block may carry data. In high mobility scenarios, when the wireless devices are moving with high velocity, the resource block is relatively small and therefore the amount of resources that may be dedicated for pilots is scarce.
The overall rate performance will then be small, either because of the large pre-log penalty of serving many wireless devices or because only a small number of wireless devices may be served to limit the pre-log penalty.